The present invention relates to a measuring system for measuring an effective value of an AC voltage of a three-phase or multi-phase, rotary-phase AC voltage, in particular in a hand-held power tool, and to a corresponding hand-held power tool. The power supply to the hand-held power tool is provided through an AC voltage, which may be subject to great fluctuations, depending on the standards and local loads on the system. Safety equipment, in particular overload cutoffs, may depend on the prevailing AC voltage. The corresponding measured values are supplied by the measuring system according to the invention.
A feeder line for a three-phase rotary phase AC current includes three external conductors and often one neutral conductor. The voltages, typically sinusoidal, on the external conductors, are each in a different phase with respect to the neutral conductor in order to supply electric power to the hand-held power tool. The reference potential of the neutral conductor can also be generated by a star connection of the external conductors within the hand-held power tool in a known way. The effective value of the sinusoidal AC voltage U is determined by a voltmeter, which records and squares the AC voltage on an external conductor with respect to the neutral conductor.
For integration into the additional circuits of the hand-held power tool, the voltmeter would expediently be designed as an electric component operated by direct voltage. However, the reference potentials, i.e., the power supply voltage and ground, would be decoupled from the AC voltage potentials, and any resulting offsets would influence the measurement of the effective value.
The hand-held power tool according to the invention has a measuring system for determining an effective value of an AC voltage supplied over an external conductor with respect to a neutral conductor. A regulated DC voltage source has a power supply potential, which is referenced to the neutral conductor. The DC voltage source adjusts the potential of a ground with respect to the power supply potential. A voltage divider is suspended between the power supply voltage and the ground. There is a first resistance value from the power supply potential to a tap, and there is a second resistance value from the tap to ground. A voltmeter has a measurement range, and its measurement input is connected to the tap. The voltmeter measures the voltage at the tap and determines the effective voltage from that. A measurement line connects the tap to the external conductor. The first resistance value is selected to be lower than the second resistance value, such that the average voltage value at the tap is equal to the middle of the measurement range for one period of the AC voltage.
The measuring system, in particular the voltage divider, can be designed and operated independently of the AC voltage fed into it. This is of interest in particular with regard to different national electric power supplies. The measurement range of the voltmeter is then utilized optimally. A possible offset based on the decoupled potentials of ground from the neutral conductor is compensated easily but nevertheless skillfully.
A voltage divider determines the effective voltage from individual measurements of the voltage at the tap. This may take place in a known manner by integration, by an analog or numerical method or by determining the maximum value of the preferably sinusoidal AC voltage and calculating the effective value based on the maximum value.
One embodiment provides that the following equation holds: R1/R2=(1−Vcc/Max), where R1 is the first resistance value (R1), R2 is the second resistance value, Vcc is the power supply voltage potential and Max is the upper limit voltage of the measurement range (Mb).
The hand-held power tool is in particular a hand-held power tool, such as an electric screwdriver, a hand-held drill, a chisel hammer, a combination hammer, a battery-operated screwdriver, a circular saw or a reciprocating saw.
The following description explains the invention on the basis of exemplary embodiments and figures.